The present invention relates generally to sensor devices, and more specifically to fluid interface position sensors which function via excitation and measurement of fluorescence.
A common problem in oil storage technologies is contamination with water. When storage depots and fuel tanks are used, it is usually desirable that only oil or fuel should be pumped from the tank to use. Unfortunately, water is denser than oil, and hence lies at the bottom of such tanks or other storage vessels. As a result, unless the storage vessel is transparent, or has a sight gauge, it can be difficult to determine how much water is in the storage vessel. Knowledge of the oil-water interface level can serve to guide attempts to remove the water without contamination or waste, to insure that the water level is sufficiently low that the fuel can be safely pumped for use, or to dynamically adjust the pumping mechanism to avoid pumping water along with fuel.
Other combinations of immiscible fluids in storage present similar problems. For example, even the level of a fluid in a tank which is otherwise filled with air presents the problem of determining the position of an interface between two fluids in a blind vessel. Additionally, there are numerous situations in chemical engineering processes where knowledge and/or control of the position of a fluid interface is required.
Many prior art sensors address the problem of measuring the position of an upper surface of a fluid. Among these include the aforementioned sight gauge, float mechanisms, sensors to detect the change in thermal conductivity between the fluid and air, sensors to detect capacitance change resulting from the change in dielectric constant when the sensor is immersed in said fluid, ultrasonic or optical time of flight sensors to detect the distance to the upper surface of the fluid, and many others.
Some of the above upper fluid level sensors can be adapted to the problem of continuously sensing the position of a fluid interface within a storage tank. In particular, the sight gauge can typically be so used without modification, although the nature of the interface must be such that it is easily seen.
Float mechanisms must be designed so that the float has density intermediate to the two fluids forming the fluid interface. However, the driving force for such a float mechanism is typically greatly reduced from that of a float sensor for surface position, a factor which makes design and operation of such fluid interface position sensors less accurate and less reliable.
However, most of these approaches are inappropriate or inadequate for many applications. Requirement for direct visual monitoring, use of moving parts in the potentially harsh environment of a storage tank, poor resolution and/or accuracy, and use of electrical sensors in applications involving flammable liquids are among the factors which illustrate the need for improved fluid interface position sensors.
Application of optical sensors to the problem of fluid interface position measurement is attractive in that such optical sensors introduce no electrical energy into the storage tank, are insensitive to electromagnetic interference, have no moving parts, are suitable for remote monitoring of the interface position, and can provide a continuous measurement of the interface position.
A number of upper surface fluid level sensors based on fiber optics have been proposed. The background of such sensors is described in U.S. Pat. No. 6,172,377, by the present inventor. Said patent is hereby included in its entirety by reference. Such sensors are typically based on the detection of cladding loss which occurs when a properly designed optical fiber is immersed in a fluid.
Another approach in the art of upper surface fluid level sensors based on fiber optics appears in U.S. Pat. No. 4,870,292, to Alpert et al. This patent teaches a fluorescent doped detector fiber which collects light reflected from a source fiber in the presence of air. This light, however, is refracted away when a fluid of sufficiently high refractive index is present. Hence, the output signal from the fluorescent doped fiber is directly related to the fluid level.
U.S. Pat. No. 4,942,306, to Colbourne, builds upon the ""292 patent above by embedding one end of the source optical fiber into a transparent substrate such that the injected light enters the substrate at such an angle that it will refract out of the substrate when the substrate is in the liquid, but will continue to be totally internally reflected when the substrate is in the air above the liquid. The detecting fiber is again a separate fluorescent doped fiber that is placed against the transparent substrate, said placement providing optical coupling between the detecting fiber and the transparent substrate.
U.S. Pat. No. 6,172,377, to Weiss, and included above by reference, describes the invention of a fluorescent optical sensor which is capable of detecting the position of a fluid interface, provided only that the upper fluid has smaller refractive index than does the lower fluid. Hence, the ""377 sensor is suitable for detection of upper surface fluid levels, air having smaller refractive index than fluids.
The ""377 sensor comprises a transparent waveguide containing fluorescent material. The fluorescent material is excited by light of a first wavelength, and responds by emission of light of a second and longer wavelength. The upper end of this waveguide is connected to a light source at the first wavelength through a beveled portion of the waveguide such that the input light is totally internally reflected within the waveguide above a fluid interface, but is transmitted into the bottom fluid below the fluid interface. (Again, within this invention this behavior requires that the upper fluid have smaller refractive index than does the lower fluid.) Hence, light is emitted from the fluorescent material only in those portions of the waveguide which are above the fluid interface. This emitted light is collected at the upper end of the waveguide by a detector sensitive only to the second wavelength. As the interface moves down in the tank, the signal strength from the detector will increase.
The ""377 sensor provides a robust, sensitive, and accurate sensor of fluid interface level, with the primary limitation that the upper fluid must have smaller refractive index than the lower fluid. However, this limitation means that the ""377 sensor cannot be applied to sensing the fluid interface level between, for example, fuel oil and water, where the upper fluid has larger refractive index than does the lower fluid.
Accordingly, there remains in the art need for a simple, low cost fluid interface position sensor based on fluorescent excitation, preferably realized as a single optical element immersed in the fluid.
A new fluid interface position sensor has been developed, which is capable of optically determining the location of an interface between an upper fluid and a lower fluid, where the upper fluid has larger refractive index than does the lower fluid. The sensor functions by measurement of fluorescence excited by an optical pump beam which is confined within a fluorescent waveguide where that waveguide is in optical contact with the lower fluid, but escapes from the fluorescent waveguide where that waveguide is in optical contact with the upper fluid.